Shifting Character Proportions
Several possible technics to do that. Of course you can move each vertex manually, but here are some possibilities that can help you (between probably many others).


Edit mode and proportional editing


Mainly two situations. Your object is mirrored or not. If it is not, you can make it mirrored (removing half of it and add a mirror modifier) or use the mirror option in the tools panel on the left here :



As you start from an obj file. I think yours is not mirrored but is symmetrical.


Enter edit mode Tab
Enable proportional editing 
Select a vertex (or more) and move it
Use the mouse wheel or the PgUp and PgDown keys you can play on the influence of the proportional editing mode and so move more or less the neighbors vertices




Other useful thing in edit mode 'smooth vertex' :




Sculpting


Enter sculpt mode and check that the symmetry is set (normally it is by default) :



Then use the brushes and sculpt ! I think you may prefer the following brushes 'draw', 'grab', 'smooth'... but test which is the best for you.
The F key allows you to modify the size of your brush.




Mesh deform


Mesh deform is a modifier. It can be interesting as it gives you the ability to deform a mesh using a more simple one.

To set it up, you'll need to make a close shape that bounds your model. And probably give it some cut in order to have some handling points. And stretch the cube to be relatively close to your model.
Here I use a cube with some subdivisions and set its maximum draw type to 'wire' so that you can see your model through it :



Next select your mesh, go to the modifiers panel, choose a 'mesh deform' and set the cube as deform object. Then click 'bind' (Blender will calculate a bit at this step) :



Once done, select the cube, enter edit mode and play with its geometry :